1. Field of the Invention
This invention is directed generally to the field of metalurgical joinder of materials and more specifically to the solid state diffusion bonding of metallic members to achieve homogeneous joints.
2. Prior Art
Diffusion bonding is an ancient metal working technique in which metal members may be joined together. Solid state diffusion bonding is basically a two-stage process. The first stage is largely mechanical in nature and involves establishment of intimate surface contact through local plastic deformation to overcome asperities at the interface point. This results in the achievement of perfect metal to metal contact in the to-be-joined area. The second stage creates the homogeneous bond by diffusion; i.e., by the movement of molecular particles of metal across the interface. During these two stages, time, temperature, cleanliness and pressure are relevant to the adequacy of the bond produced. This technique for joining metal has been disregarded by structural design engineers for many years because of their inability to design readily buildable structural components. As a result, no significant saving of material could be achieved which would offset the increased costs incurred in utilizing the diffusion bonding process. Today, however, solid state diffusion bonding is in great demand for utilization in the aerospace industry among others, because it enables the achievement of homogeneous joints of parent material properties with no residual joining stresses. More precise knowledge of fabrication techniques enables sufficient savings in material to be made to warrant the higher costs of fabrication. Thus, where the strength to weight ratio is a paramount consideration, the diffusion bonding process provides the ability to fabricate near optimum shaped structural components from efficient material thereby achieving a reduction of structural weight while maintaining the required structural integrity. Diffusion bonding is almost without limit in its versatility and more than any other known method, affords the designer a great freedom to design new and different structural configurations and maximize the strength to weight efficiency.
In general, in addition to cost, three major factors must be taken into account when designing any type of structure. First is structural integrity, second is minimum weight and third is the ability to fabricate a real structure, duplicating the theoretical structural model and boundary conditions. These factors force the structural engineer to improve the accuracy of his analysis while developing arrangements of structural elements made of high strength materials joined in a manner to achieve higher structural efficiency. In these design analyses, any deviation from the theoretical homogeneous geometry causes locked in stresses, joint discontinuities and variable strain rates, resulting in lower structural efficiency. In addition, such deviations often result in weight penalties through the appearance of stress risers at irregular joints or connections. The diffusion bonded method of forming connections allows fabrication duplication of a theoretical design configuration which depends on a homogeneous material at the juncture points. The process also eliminates locked-in stresses and joint discontinuities.
The aerospace industry is the largest user of the solid state diffusion bonding process because of its ability to use complex analytical techniques to achieve highly efficient structural concepts. The designed structure can first be refined to an ultimate strength to weight efficiency and then mass produced. Airborne equipment requires that structural concepts yielding maximum strength and stiffness to weight ratios be utilized to achieve the required power to weight efficiency. In all cases it is desirable to minimize the weight of the structure. The aerodynamic gas pressure and flow characteristics typically impose bending strength/stiffness, torsional strength/stiffness, and shear strength/stiffness requirements upon the structural configuration that must be considered in the design. Diffusion bonding allows design of a joining technique which permits maximum strength to weight ratio structures.
In diffusion bonded joints an important aspect is the placement of the transition material for the efficient load transfer from the smaller load carrying member to the more massive member. Ideally, the smaller load carrying member and the larger member should be completely homogeneous members through out. One approach to this ideal structure is to carve the most efficient structural configuration from a mass of homogeneous material having a high strength to weight ratio, thereby, precluding any need for joining. However, except for the most simple structural components, the one piece homogeneous structure is neither optimally weight efficient nor economically feasible. The known techniques for producing homogeneous structural components include machining from bar or plate stock, net forging, forging plus machining and extruding (for constant section members). However, these production techniques are often not feasible when irregularly shaped, massive solid members at random locations are required, and they are especially infeasible when small internally stiffened shell members are required.
Because of the limitations on producing one piece homogeneous structures, techniques for joining one member to another are typically used in the prior art. All prior art production methods of joining metals, such as, for example, riveting, bolting, welding, brazing, organic bonding and polyimide bonding result in a load transfer capability lower than that of the parent material utilized; i.e., they do not achieve the required homogeneous properties and strain rate of the parent material across the joint. Welding, brazing and organic bonding are not only incapable of achieving the required homogeneous properties of the parent material across the joint but can not be used on complex shapes. Casting with alloys is not competitive for light-weight structurally efficient parts when compared to the wrought alloys. Extrusion can be used for any linear shape but cannot be performed when complex variable shapes are involved. The only real alternative is machining, which cuts the net shape from a solid piece of metal. This method, however, is costly, and if thin members of complex shape are desired, machining becomes very inefficient. It is also expensive because of the amount of alloy which must be machined from the original piece of metal.
The solid state diffusion bonding technique, on the other hand provides, (i) a means for achieving full parent material strength across the joint interface because no foreign material is utilized, and (ii) strain compatibility across the joint interface. Several diffusion bonding techniques have been developed, such as, for example, roll bonding, press bonding and vacuum bag bonding. However, each of these techniques imposes limitations on the structural configuration achievable. For example, roll, press and vacuum bag diffusion bonding techniques cannot produce the desired blending or filleting. In addition, they are not applicable to complex aerodynamic shapes. Each of these known techniques of diffusion bonding and their respective limitations and shortcomings are briefly described hereinbelow.
The roll diffusion bonding technique utilizes a steel tooling retort with positioning filler tooling to locate the members to be joined in proper respective positions. The intimate contact is established by roll reducing both the retort tooling and the to-be-joined parts by a percentage (generally 50% to 60%) to guarantee completely intimate surface contact and diffusion bonding. Thus, this process requires expendable tooling and is basically limited to the attachment of members in the rolling direction. The degree of joined member filleting is limited by the combination of tooling material flow and detail parts flow.
The press diffusion bonding technique utilizes reusable positioning and restraining tooling and massive hydraulic presses as the pressure source to establish the intimate contact. However, in order to utilize reusable tooling, the local surface deformation is generally limited to less than 5%. This requires the surfaces to be joined to be matched within very close tolerances. In addition, it also requires, because of the relatively low local unit pressure, a long time at the elevated temperature to allow the diffusion cycle to complete. Flow filleting is very limited because of the low local deformations allowable.
The vacuum bag bonding technique utilizes atmospheric pressure as the pressure source and is thus limited to very thin sheet structures which can attain the required intimate surface contact at this relatively low pressure. Because of this low pressure, a very long time at the elevated temperature is required to complete the diffusion cycle. In addition, flow filleting cannot be achieved.
The present invention overcomes these above-described limitations of the prior art and discloses a method of producing a diffusion bonded joint between a first member and a second, more massive member using minimum energy principles, i.e., minimum energy or force is required, sufficient only to flow the material at each juncture on the members. The invented method for diffusion bonding utilizes reusable tooling and achieves filleting and attachment in multiple directions. It also enables one to shape the member intersections so as to minimize stress concentrations. Moreover, all of the advantages of the present invention are attainable within the constraints of the economic feasibility.